LONDON, England (CNN) -- Air crash investigators are trying to work out why a Boeing 777 landed short of the runway at London Heathrow airport, skidding on grass and ripping apart sections of the aircraft.

I-Reporter Alex Quinonez took this image of a casualty being taken by medics from Heathrow Airport.

An investigator who has been briefed on the incident told CNN the plane's captain "is claiming there wasn't power when he needed it."

Passenger Paul Venter told the UK Press Association: "The wheels came out and went for touchdown, and the next moment we just dropped. I couldn't tell you how far."

London ambulance services said 17 people suffered minor injuries, and the number could increase as several others are still being assessed.

Images showed the Boeing 777 -- BA flight 38 from Beijing, China -- grounded on tarmac after touching down several hundred meters short of the airport's south runway, close to a perimeter road, with its emergency chutes deployed and white fire-fighting foam covering the engines.

The undercarriage, left wing and left engine of the aircraft were severely damaged, as if it had skidded across the ground. At least one of the plane's wheels had been torn off.

The most visible damage was to the left wing, which was covered in mangled metal where it meets the fuselage.

Tire tracks hundreds of meters long could be seen in the grass behind the plane, which was surrounded by fire engines and other emergency vehicles.

Eyewitness Neil Jones said the plane had made a "very, very unusual approach" to the airport and sounded louder than usual, PA reported.

The BBC said an unidentified Heathrow worker told the broadcaster that he had spoken to the pilot. The pilot said, according to the worker, that the plane's electronics had failed and that he was forced to glide it to the ground.

The UK Air Accident Investigation Branch will lead the inquiry into the crash landing. A team from the U.S. National Transportation Safety Board is also heading to London, accompanied by representatives from Boeing and the Federal Aviation Adminsitration.

Jerome Ensinck, a passenger aboard the flight, said there had been no indication that the plane was making an emergency landing.

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"There was no indication that we were going to have a bad landing," he said. "When we hit the ground it was extremely rough, but I've had rough landings before and I thought 'This is the roughest I've had.'

"Then the emergency exits were opened and we were all told we should go through as quickly as possible, and the moment I was away from the plane I started to realize that the undercarriage was away, and we had missed the runway.

"I feel lucky at the moment, but I think now I realize I've had a close call. If we had hit the runway, it would have been worse."

In a statement, British Airways said all 136 passengers and 16 crew members had been evacuated from the plane with six minor injuries taken to hospital.

BA chief executive Willie Walsh praised the actions of the crew. "We are very proud of the way our crew safely evacuated all 136 passengers on board," Walsh said in a statement.

"The captain of the aircraft is one of our most experienced and has been flying with us for nearly 20 years," he added.

Walsh also said that an investigation was being conducted by the Air Accidents Investigation Branch and that it would be inappropriate to speculate about likely causes.

Airport authorities said Heathrow's southern runway had been closed, but the northern runway remained open. But the incident immediately led to major delays for passengers. Some incoming flights were being diverted to other airports on a flight-by-flight basis, according to Heathrow's Web site.

A spokesman for London's Metropolitan Police said there was nothing to suggest the incident was terror-related.

The Boeing 777 is the mainstay of many airlines' long-haul fleets and has never been involved in a fatal accident. However, the aircraft involved in Thursday's incident appeared to have had a fortunate escape, having approached Heathrow over heavily-populated west London suburbs before its crash landing.

CNN's Richard Quest, who covers the airline industry, said it appeared the damage happened after the plane touched down.

The incident occurred at 12:42 p.m. (7:42 a.m. ET) as British Prime Minister Gordon Brown was due to leave Heathrow for a visit to China and India. His flight was delayed but his jet was not directly involved, PA said.

• British Airways has set up helpline numbers for friends and relatives concerned for passengers involved in the incident: